1. Field of the Invention
The present invention relates to analysis of the relative power contributions of individual cylinders of an internal combustion engine and, in particular, relates to techniques for determining such power contributions from variations in the engine speed.
2. Description of the Prior Art
Heretofore, in order to analyze the relative power contributions of individual cylinders of a multi-cylinder internal combustion engine, a technique known as "cylinder shorting" was used. In this technique, the contribution of an individual cylinder to the overall power output of the engine is determined by "shorting", or preventing the occurrence of the ignition event of a cylinder and noting the resultant change in engine speed in rpm. An ignition event is the firing of the spark plug in the case of a gasoline engine or the fuel injection in the case of a diesel engine. If the cylinder were contributing no power, then the shorting of that cylinder would not decrease the engine speed. If, on the other hand, the individual cylinder were a normal contributor to the overall power then, when it is shorted, the speed of the engine would drop. If each individual cylinder normally contributed a like amount to the overall engine power then, when it is shorted, each cylinder would cause the same drop in engine speed.
In modern computer-controlled engines, with catalytic converters, it is difficult and potentially harmful to short out cylinders. Accordingly, alternative techniques of measuring the cylinder power balance have been developed. Thus, it has been found that the speed variation of the engine within a single engine cycle can give an indication of the cylinder power balance. In normal operation, each time a cylinder ignition event occurs, the power stroke of that piston causes the engine to accelerate momentarily. The engine speed increases to a peak and then begins to decrease as the engine "coasts" until the ignition event for the next cylinder in the firing order. Thus, in a normally operating engine, a plot of the engine speed against time over a single engine cycle is in the shape of a generally sinusoidal waveform with one period or cycle of the waveform for each cylinder. If all cylinders are contributing equally to the overall engine power, the peaks of this waveform should all be at substantially the same height.
One system for determining power balance by analyzing engine speed variations over a single engine cycle is disclosed in U.S. Pat. No. 4,539,841. In this system, the engine speed is sensed by use of an electromagnetic sensor positioned adjacent to the teeth on the engine flywheel or ring gear. The sensor senses the passing of the teeth of the ring gear as it is rotated and provides an alternating output, the frequency of which is proportional to the engine speed. While this permits fairly accurate monitoring of engine speed, it has the disadvantage that it requires measurement of the absolute engine speed. Furthermore, the installation of the sensor is very cumbersome and time consuming. The ring gear is relatively inaccessible in most engines and the installation of the electromagnetic sensor is critical. It must be very precisely positioned relative to the gear teeth and this position must be accurately maintained in order for the system to work properly.